Electrode configuration for electrophotography

ABSTRACT

A persistent internal polarization (PIP) electrophotography printing or copying system wherein one electrode is nonremovable and has a discontinuous configuration, such as a wire screen mesh, which is embedded in or attached to the surface of a PIP layer and the other electrode is transparent so that each electrode is capable of simultaneously applying an electric field to the PIP layer, while permitting radiation to reach the PIP layer.

United States Patent [72} Inventors Felix H. Brown East Lansing; RobertClark, Mason, both of. Mich. 121] Appl. No. 764,683 [22] Filed Oct. 3,1968 [45] Patented Aug.3,l97l I 73] Assignee Owens-Illinois, Inc.

[54] ELECTRODE CONFIGURATION FOR ELECTROPHOTOGRAPHY 2 Claims, 3 DrawingFigs.

[52] U.S. Cl ..l 355/3, 96/1 A, 340/173 PP [51] Int. Cl 603g 15/00 [50]Field oiSearch 355/3.17, 16,12;96/1.5,1;340/173 PP [56] References CitedUNITED STATES PATENTS 2,912,592 11/1959 Mayer 96/l.5

noon: us MDIAT'I on) 3,005,707 10/1961 Kallmann et a1 96/1.5 3,146,6889/1964 Clark et al. 355/16X 3,199,086 8/1965 Kallmann et a1... 355/17X3,268,331 8/1966 Harper 355/17 X 3,288,602 11/1966 Snelling et a1. 355/3X 3,457,070 7/1969 Watanabe et al. 96/1 .5 X

Primary Examiner-Samuel 5. Matthews Assistant Examiner-Robert P. GreinerAttorneys-Alan J. Steger and E. J. Holler ELECTRODE CONFIGURATION FORELECTROPIIOTOGRAPI'IY BACKGROUND OF THE INVENTION This invention relatesto novel apparatus and process for practicing electrophotographicprinting or copying. More. particularly, this invention relates toprinting apparatus and process utilizing photoconductive insulatingmaterials and the principles of persistent internal polarization.

Persistent internal polarization (abbreviated herein as PIP) involvesthe separation of positive and negative charges in a photoconductiveinsulating material by subjecting it to irradiation and an electricfield. The charges are subsequently trapped and remain fixed or frozenso as to form an internal polarization field for a period of timesufficient to permit toning. PIP and the theory thereof are well knownin the electrophotography art. See, for example, Electrophotography, byR. M. Schaffert, The Focal Press, London and New York (I965), pages 59through 77, and Persistent Internal Polarization, by Kallmann andRosenberg, The Physical Review, Volume 97, No. 5 Mar. l5, I955), pagesI596 through l6l0, both ofwhich are incorporated herein by reference.

In general, a PIP electrophotography system includes a layer ofphotoconductive insulating material sandwiched between a pair of fieldproducing electrodes. The phenomenon of PIP can be achieved in anymaterial which exhibits the following characteristics:

1. The material must have a high resistivity in the dark (a low densityof free charge carriers), whereby it is a good insulator in the absenceofirradiation.

2. The material must be photoconductive. In other words, it must havedecreased resistivity when excited with appropriate irradiation.

Thus, a PIP material is one which will become persistently internallypolarized due to the separation of positive and negative charges when itis subjected to irradiation and the action ofan electric field.

Typical PIP materials contemplated herein comprise binder dispersions ofphotoconductors and binder free thin films of photoconductors.

Examples of inorganic photoconductors contemplated in the process ofthis invention include, not by way of limitation, appropriatelyactivated zinc sulfide, cadmium sulfide, zinc selenide, cadmiumselenide, cadmium oxide, zinc-cadmium selenides, and zinc-cadmiumsulfides. Examples of organic photoconductors include anthracene,chrysene, and poly (vinylcarbazole).

Examples of resin binders contemplated herein include, not by way oflimitation, cellulose acetate, cellulose ether, cellulose ester,silicones, vinyl resins, alkyds, and/or epoxy resins.

When it is desired to form a latent electrostatic image in the PIPmaterial, it is flooded with radiation and an electric field is appliedso as to polarize the PIP layer. After termination of the floodingradiation, the polarity of the electric field across the PIP material isreversed and the PIP materials exposed to an image or other pattern ofactivating radiation. The reversal of the electric field will causerapid depolarization of that portion of the PIP material renderedphotoconductive under the influencc of the imagewise radiation.

If the exposure to the image is continued for a sufficient time period,the irradiated area of the PIP layer will repolarize and assume apolarization opposite to that of the nonirradiated or dark portion ofthe PIP layer. Thus, the image is simulated by an internal latentelectrostatic image or pattern detectable at the surface of the PIPmaterial.

This latent electrostatic image is subsequently developed with chargedor dipolar toner particles so as to produce a visible reproduction ofthe image which is capable of being viewed, photographed, ortransferred, utilizing known methods in the electrophotography printingor copying art.

It should be noted that, due to the characteristics of the PIP material,the latent electrostatic image produced in the PIP material willtypically remain fixed such that a finite number of reproductions can bemade. The image can be erased by overall irradiation with or without anelectric field, thereby returning the PIP material to a prepolarized orneutral condition capable of being used for the formation of a newelectrostatic image.

Theirradiation of the PIP material (for polarization and/or imaging) canbe accomplished by means of any form of electromagnetic or-particulateradiation or energy, visible or invisible, which will excite the PIPmaterial so as to permit charge separation in an electric field. Suchradiation includes, not by way of limitation, visible light, infrared,ultraviolet, X- rays, gamma rays, and beta rays. For printing or copyingpurposes, the typical radiation is light in the visible range.

In the prior electrophotographic printing and copying art, simultaneousapplication of the electric field and the light from an image to a PIPmaterial has been obtained by means of at least one continuous electrodewhich is substantially transparent. The subsequent'development andtransfer of the electrostatic image using electroscopic powders orliquids has to date required that the continuous electrode be removable.

A nonremovable, discontinuous wire mesh electrode has been introducedwhich does not need to be removed for toning and transfer purposes.However, since such a discontinuous electrode is not completelytransparent, it impedes the impingement of light on the PIP material inthe regions of the strongest field, thereby slowing the polarization andimage formation in the PIP material. This speed of polarization andimage formation becomes particularly important when it is desired to usea PIP system in a printing or copying machine.

SUMMARY OF THE INVENTION In accordance with this invention, there isprovided a novel PIP system which includes both a nonremovable electrodeconfiguration, which does not have to be removed for image developmentand transfer purposes, and a completely transparent electrode whichpermits total impingement of light radiation on the PIP layer tofacilitate rapid polarization and image formation within the PIP layer.

In a specific practice hereof, this invention features the combinationof a thin mesh grid electrode placed on or embedded in the top surfaceof a PIP layer and a transparent electrode, such as a tin oxide-coatedglass. Thus, a sandwich configuration is achieved whereby the baseelectrode is a conductive glass, the PIP layer in a suitable binder isapplied on top of the base electrode to a suitable thickness, and themesh electrode is placed on top of the PIP layer and embedded in orattached to the PIP layer.

In operation, an electric field is applied between the conductive glasselectrode and the embedded mesh electrode and the flooding radiation isimpinged upon the PIP layer through the transparent conductive glasselectrode. The imaging radiation may then be impinged upon the PIP layerthrough either the conductive glass electrode or the embedded screenmesh electrode. Formation of an image occurs more rapidly when the imageradiation is impinged upon the PIP layer through the conductive glasselectrode than it does when passed through the embedded screen meshelectrode. However, successful image formation can also be accomplishedwhen the image radiation is passed through the embedded screen meshelectrode as it contains sufficient openings (60 to percent) to besubstantially transparent.

Other features and advantages of the subject invention will becomeobvious to those skilled in the art upon reference to the followingdetailed description and'the drawings illustrating a preferredembodiment of the invention.

IN THE DRAWINGS FIG. 1 is a schematic view of a PIP system having both anonremovable, discontinuous electrode and a transparent conductive glasselectrode in accordance with this invention.

FIG. 2 is a schematic view of the PIP system of FIG. 1 upon theformation of a latent electrostatic image.

FIG. 3 is a schematic v e of a nonremovable, discontinuous electrode inthe form of a flat wire mesh embedded in the surface ofa PIP layer.

DESCRIPTION OF A PREFERRED EMBODIMENT In the drawings, the numeral 10refers to a body of PIP material as previously described. The PIP body10 is sandwiched between a pair of electrodes 12 and 14 which areconnected to a DC source E. For the purposes of explanation, theelectrode 12 is connected to the positive terminal of the DC source Eand, accordingly, the electrode 14 is connected to the negative terminalof the DC source E.

The electrode 12 is a transparent, conductive glass electrode and may,for example, be a tin oxide-coated glass.

The electrode 14 is discontinuous and may take the form of a mesh whichis partly or entirely embedded in the top surface of the PIP body 10.The electrode 14 should be sufficiently embedded in the top surface soas to be flush with the outer face thereof. One possible such meshmaterial would be an electroformed nickel mesh which can be obtainedcommercially as fine as 2,000 lines per inch. The light transmission ofsuch mesh may be varied by controlling the space and wire dimensions.

To initially prepolarize the PIP body 10, the system is flooded withlight through the conductive glass electrode 12 as shown in FIG. 1.Under the combined action of the light and the DC source E, it is shownschematically that negative charges are effectively conducted to theedge of the PIP body 10 adjacent to the electrode 12 connected to thepositive terminal of the DC source E, and, conversely, positive chargesare effectively conducted to the edge of the PIP body I adjacent to theembedded mesh electrode 14 which is connected to the negative terminalof DC source E.

When the system is subjected only to imagewise radiation while thepolarity ofthe DC source E is reversed, the PIP body reacts as shown inFIG. 2. In FIG. 2 it is seen that only those areas of the PIP bodysubjected to the imagewise radiation undergo internal polarization underthe force of the field produced by the reversed polarity of source E.The PIP system has thus produced a latent electrostatic image (asrepresented schematically by the middle four negative charges on theright side of PIP body 10 adjacent to embedded mesh electrode 14 in FIG.2) which is capable of being toned and transferred through the use ofcharged electroscopic particles (not shown). As previously described,the image-wise radiation may be impinged upon the PIP layer I0 througheither the conductive glass electrode 12 or the embedded mesh electrode14.

A continuous electrode has an inherent disadvantage in that it must beremoved in order to develop the latent electrostatic image as it acts asa shield between the latent electrostatic image and the electroscopicparticles because of the overall presence of image charges. In contrastto this, the discontinuous electrode (embedded mesh electrode 14) ofthis invention with its high percentage of openings appreciably reducesthe shielding of the electroscopic particles from the latentelectrostatic image and, therefore, need not be removed during thetransfer or printing stages. Thus, a discontinuous electrode has adistinct advantage in that its nonremovability saves considerable timeand facilitates the transfer speed necessary to successfully use a PIPsystem in a printing or copying machine.

In commonly available copying machines, it has been found that thetoning of a large solid area often results in decreased toner density;that is, deterioration of the image in areas furthest away from theedges. In other words, the middle portions of a large solid area whichhas been toned and transferred from such an image often appear lessdistinct than do the edge portions. The use of a discontinuous electrode(such as a mesh) electrically breaks up the large areas, therebyresulting in uniform development over the large solid areas of thelatent image.

An advantage of a nonremovable electrode, such as embedded meshelectrode 14, is its ability to avoid dust collection between theelectrode and the PIP layer. Removable electrodes frequently pick updust particles and other foreign matter which, when positioned betweenthe electrode and the PIP layer, distort the effect of the field linesof the PIP layer. A nonremovable electrode embedded in the top surfaceof the PIP layer such as the mesh of this invention completelyeliminates the possibility of dust particles gathering between theelectrode and the PIP layer, thereby insuring against distortion of thefield lines.

Another advantage accrues in this PIP system. The possibility existsthat in transferring toner from the PIP drum to the substrate to beprinted, the application of the electric field which transfers theelectrostatically charged powder may deteriorate the PIP image and thusminimize the possibility of repeatedly toning and transferring from asingle imaging. With the embedded screen mesh of this invention, thispotential difficulty is completely eliminated. For this purpose, theelectric field used to transfer the toner particles to a substrate to beprinted is applied to both the mesh electrode 14 and the transparentelectrode 12 so that both electrodes are at the same potential. Theother electrode is placed on the opposing side of the substrate to beprinted. This produces an electrostatic field between the embeddedscreen mesh and the substrate, which moves the toner particles to thesubstrate surfaces. It also provides that there will be no applied forcewithin the PIP layer and thus eliminates any degradation effect whichtransfer electric fields frequently cause.

Given the combination of the nonremovable, embedded mesh electrode andthe transparent conductive glass electrode of this invention, there area number of variations which can be utilized within the scope of thisinvention. All of the presently known methods of polarizing the PIPmaterial can be used, including the image reversal method, as well asthe direct polarization method. Either an electrostatic tonerdipolar-type a dipolartype toner can be used. The embedded meshelectrode can be of a screen mesh either woven or electrically formedmesh with various thickness to aperture ratios; or it can be of metalevaporated onto the PIP layer surface by standard evaporizationtechniques. The transparent electrode 12 may be formed of anytransparent glass or plastic with a conductive coating, such as tinoxide, deposited thereon.

Thus, the invention as described herein provides the combination of anonremovable electrode, which can be used to simultaneously apply anelectric field and permit the imaging radiation to reach the PIP layerand which does not have to be removed for image transfer and printingpurposes, and a transparent conductive coated electrode, which iscompletely transparent to allow rapid transfer of light to the PIP layerto facilitate fast polarization of the PIP layer. It should be notedthat although this invention has been described in connection with aplanar system, it is well suited to be used in conjunction with a rotarydrum system.

Although but one embodiment of the subject invention has been shown anddescribed in detail, it should be clear to those skilled in the artthat, in accordance with the preceding description, many changes andmodifications may be made thereto without departing from the scope ofthis invention. Therefore, this invention is not intended to be limitedexcept as defined in the claims hereinafter.

We claim:

1. A PIP electrophotographic printing or copying machine wherein apersistent electrostatic latent image is formed in a photoconductivebody exhibiting persistent internal polarization when sandwiched betweena pair of conductive electrodes wherein an electric field exists betweenthe electrodes and through the photoconductive body when floodingradiation is impressed on said photoconductive body through one of saidpair of electrodes and then an image is impressed on saidphotoconductive body through the other of said pair of electrodes whilereversing the polarity of said electric field, said one of saidelectrodes comprising a completely transparent plate of glass with aconductive coating thereon, said other of said electrodes comprising asubstantially transparent foraminous wire screen mesh, and each of saidelectrodes remaining adjacent to and in contact with saidphotoconductive body during toning and transfer of said electrostaticlatent image from said photoconductive body.

2. A PIP electrophotographic printing or copying machine wherein apersistent electrostatic latent image is formed in a photoconductivebody exhibiting persistent internal polarization when sandwiched betweena pair of conductive electrodes wherein an electric field exists betweenthe electrodes and through the photoconductive body when floodingradiation is impressed on said photoconductive body through one of saidpair of electrodes and then an image is impressed on saidphotoconductive body through the other of said pair of electrodes whilereversing the polarity of said electric field, said one of saidelectrodes comprising a completely transparent plate of glass with aconductive coating of tin oxide thereon,- said other of said electrodescomprising s substantially transparent foraminous wire screen mesh whichis completely embedded in the surface of said photoconductive body, andeach of said electrodes remaining adjacent to and in contact with saidphotoconductive body during toning and transfer of said electrostaticlatent image from said photoconductive body.

1. A PIP electrophotographic printing or copying machine wherein apersistent electrostatic latent image is formed in a photoconductivebody exhibiting persistent internal polarization when sandwiched betweena pair of conductive electrodes wherein an electric field exists betweenthe electrodes and through the photoconductive body when floodingradiation is impressed on said photoconductive body through one of saidpair of electrodes and then an image is impressed on saidphotoconductive body through the other of said pair of electrodes whilereversing the polarity of said electric field, said one of saidelectrodes comprising a completely transparent plate of glass with aconductive coating thereon, said other of said electrodes comprising asubstantially transparent foraminous wire screen mesh, and each of saidelectrodes remaining adjacent to and in contact with saidphotoconductive body during toning and transfer of said electrostaticlatent image from said photoconductive body.
 2. A PIPelectrophotographic printing or copying machine wherein a persistentelectrostatic latent image is formed in a photoconductive bodyexhibiting persistent internal polarization when sandwiched between apair of conductive electrodes wherein an electric field exists betweenthe electrodes and through the photoconductive body when floodingradiation is impressed on said photoconductive body through one of saidpair of electrodes and then an image is impressed on saidphotoconductive body through the other of said pair of electrodes whilereversing the polarity of said electric field, said one of saidelectrodes comprising a completely transparent plate of glass with aconductive coating of tin oxide thereon, said other of said electrodescomprising s substantially transparent foraminous wire screen mesh whichis completely embedded in the surface of said photoconductive body, andeach of said electrodes remaining adjacent to and in contact with saidphotoconductive body during toning and transfer of said electrostaticlatent image from said photoconductive body.